The invention relates to a method and circuit for displaying at least two analog signals via a band display. The display scale consists of several successive display elements, of which the component defined by an analog signal in each scanning cycle is made luminous by a specific scanning frequency to produce an analog display.
The method and circuit of the invention are especially applicable to a visual display of two uninterrelated analog signals on one display scale of a gas discharge tube of the Burroughs SELF-SCAN type. A gas discharge tube of this type includes, for example, a neon-filled, two-branched flat, long glass envelope having opposite walls equipped with electrodes. The cathodes of the tube are divided into several components according to the various functions. The components include a reset cathode and three or five comb-like phase cathodes, the branches of which are sequentially interleaving. Each scale has its own anodes.
The anodes opposite the phase cathodes are transparent so that when a sufficient voltage is applied between an anode and a cathode, a gas discharge can be seen through the front glass of the scale as a light phenomenon. A discharge of this type may be started at the lower edge of the discharge tube scale. An anode and the first branch of the first phase cathode then locate the light phenomenon. The first branch of the second phase cathode is the next in sequence. When the control current is switched to the second phase cathode and the first phase cathode is disconnected, the discharge moves one step forward. One scale usually contains 201 step elements.
Known control circuits may produce on each scale of the gas discharge display tube a display of one analog signal for usually two-scaled display tubes. The function of the control circuit is to connect anode/cathode pairs in succession so that the discharge covers the whole scale within one operating cycle, if necessary. The propagation of the discharge can be stopped, if necessary, by disconnecting the anode current, for example. Scans of a sufficiently high frequency may be repeated past a selected number of sub-divisions to produce an analog display so that the eye sees the scan as a continuous display. The number of sub-divisions may be made comparable to the signal to be displayed as follows.
From a specific instant in time, the voltage of a linear ramp generator is permitted to increase and a phase generator controlling the cathodes of the discharge tube is simultaneously permitted to connect phase cathodes in succession to propagate the gas discharge on the scale. The ramp voltage is compared with the analog signal voltage in a comparator circuit. As soon as the ramp voltage reaches the analog signal voltage, the display of the discharge tube is switched off. A visual display of the analog signal is obtained by repeating the process.
The aforedescribed known method permits the display of the discharge tube to be made simultaneously at the two display branches, or in turn, by using separate anodes and controlling them.
If a gas discharge tube is used as a signal display unit of an analog process regulator, the problem of displaying three signals to the operator has to be solved. The known solutions are usually based on the display of two signals, a measurement point and a set point, on two scales of the discharge tube, as well as on the display of the starting signal in a separate indicator or display device. In the use of the regulator, the comparison of the measurement point and the set point is relevant. If the displays of these two values are on separate scales, a rapid comparison is difficult. If it is preferred to display two values on the same scale of the gas discharge tube, a solution must be found for obtaining a sufficient difference to identify the displays, so that such displays will not mix with each other during observation.